Group-directed communications are commonplace in enterprise and public safety communication systems. With regard to voice communications, one end device directs an audio stream to a given group (i.e. a “talkgroup”) of receiving end devices. These receiving end devices reproduce the audio stream through an amplified speaker. The manner in which the receiving end devices are used usually results in the reproduced sound being audible to people other than merely the intended recipient. Typically, the receiving end devices are often located near each other, causing their associated listeners to hear the same audio stream simultaneously reproduced by multiple end devices. This is particularly true in public safety uses, in which personnel often respond to incidents in a group and this group (or a subset thereof) may be located in the same local area for an extended period of time.
In order to ensure the audio stream is intelligible to the intended listeners in such an environment, it is desirable for collocated devices to reproduce the audio stream in a time synchronized fashion. In other words, all amplified speakers in the collocated devices should reproduce the same audio waveform at roughly the same instant in time. In general, a temporal offset of at most 30 ms between multiple audible speakers reproducing the same waveform is virtually undetectable to most listeners. Modern wireless voice communication systems achieve synchronized presentation of group-directed audio through an over-the-air simulcast of circuit-switched audio at multiple transmitting sites. Dense populations of collocated end devices thus receive the same over-the-air signal at roughly the same instant in time.
Such methods of synchronized presentation work well for the specialized homogeneous narrowband circuit-switched wireless radio networks typically used in the current generation of enterprise and public safety communication systems. However, the next generation of such communication systems is likely to span multiple narrowband circuit-switched and broadband packet-switched Radio Area Network (RAN) technologies with wholly different methods of synchronization. Example circuit-switched narrowband RAN technologies include 25 kHz, 12.5 kHz, or 6.25 kHz equivalent FDMA or TDMA air interfaces (e.g. Project 25, TETRA, DMR). Example packet-switched broadband RAN technologies include LTE, UMTS, EVDO, WiMAX, and WLAN air interfaces. Without a mechanism to synchronize media reproduction in a communication system comprised of heterogeneous RAN technologies, end devices connected to the circuit-switched narrowband RAN and end devices connected to the packet-switched broadband RAN would reproduce the same audio waveform in an autonomous fashion with respect to one another. This cacophony of misaligned sound results in unintelligible audio communication where multiple narrowband and broadband end devices are collocated.
Additionally, half-duplex group communication systems provide a mechanism to ensure equitable speaking rights on a given shared communication resource such as a channel or “talkgroup.” To provide this, the floor (i.e. the right to broadcast) is typically granted to the first device to make an appropriate request. During a half-duplex group conversation, listeners wait for the current audio stream to finish before initiating a new floor request. If the floor is granted to the first requester, it is desirable that all listeners be given the opportunity to request the floor at the same instant. This can be achieved if the preceding audio stream ends at the same time for all listeners.
In addition to potential intelligibility problems, without synchronized audio reproduction, the same audio stream will terminate at different times for listeners whose end devices are connected via different RAN technologies. Since floor control is typically granted to the first requester, end devices whose reproduced audio stream is lagging are not given equal rights for floor acquisition. Thus, it is desirable to provide a mechanism to synchronize audio reproduction across end devices operating on heterogeneous RANs.